I do not have an NFT system but with my DWC , drip system ,my ebb and flow system , or even my Kratky still water system. What I do is always monitor my nutrient levels and PH levels and top up to the desired levels nominated.

For my DWC and Kratky systems i do the same but I do watch the levels in relation to root growth as I do not want to drown the roots, so I fill up with the nutrient solution with the correct PH and EC levels to just below the plant cups so as that oxygen still gets around the roots and no drowning occurs. In other words I do the same for my DWC at initial seedling stage where the roots just touch the nutrients and continue down.